High intensity discharge (HID) lamps contain an ionizable fill, or discharge plasma, that emits visible radiation when the fill is sufficiently excited; such excitation typically results from passage of electrical current through the fill. Typical fills include a medium to high pressure ionizable gas, such as mercury or sodium vapor. The present invention is directed to a class of HID lamps comprising electrodeless lamps that induce an arc discharge in a high pressure gaseous fill by generating a solenoidal electric field in the fill. The lamp fill is excited by radio frequency (r.f.) current in an excitation coil proximate a main arc tube.
Difficulties have arisen in achieving a full, or steady state, discharge in HID lamps of the foregoing type, i.e. those powered from an excitation coil proximate a main arc tube. One reason is that, at room temperature, the solenoidal excitation produced by the excitation coil is typically too weak to ionize the lamp fill and initiate arc discharge. Prior art approaches to this difficulty include the provision of starting aids that directly or capacitively couple high voltage to the main arc tube. The voltage gradient created in the main arc tube ionizes the lamp fill in progressive stages, from a relatively low current glow discharge, to a relatively high current, high intensity solenoidal arc discharge.
A typical prior art starting aid that directly couples high voltage to the main arc tube is disclosed in U.S. Pat. No. 5,047,693 of V. D. Roberts et al., issued Sep. 10, 1991; and a typical prior art starting aid that capacitively couples high voltage to the main arc tube is disclosed in U.S. Pat. No. 5,140,227 of J. T. Dakin et al., issued on Aug. 18, 1992. The foregoing patents are assigned to the instant assignee and hereby incorporated by reference.
While such starting aids have improved the starting ability of HID lamps of the mentioned type, other difficulties in starting such lamps are still typically encountered. For instance, lamp starting typically requires repeated tuning of a network for matching the impedance between the lamp excitation coil and its r.f. power source. Such impedance-matching network is tuned for the period of starting so as to achieve substantially full power transfer to the excitation coil. Once the lamp has reached full, or steady state, arc discharge, the impedance of the lamp excitation coil changes, requiring the impedance-matching network to be re-tuned to again achieve substantially full power transfer to the coil. Such tuning and re-tuning, moreover, is often a delicate procedure not readily susceptible to reliable automation.
It would thus be desirable to provide a system for automatically starting an HID lamp of the mentioned type that overcomes the need for the often-delicate tuning and re-tuning of an impedance-matching network to start and operate a lamp.
A further difficulty results in attempting to restart a lamp after being extinguished from steady state operation. A lamp may be extinguished accidentally, for instance, if a momentary disconnection of the lamp from its power supply circuit occurs. It has been found that the lamp typically cannot be re-started until it thermally recovers, i.e. cools below a critical level necessary for restarting to occur. For a 300-watt lamp, for instance, the thermal recovery period is typically on the order of 10 minutes, depending, however, upon the ambient thermal environment and construction of the lamp.
It would thus be desirable to provide a system for automatically starting an HID lamp of the mentioned type that overcomes the difficulty faced in re-starting a lamp after being extinguished from steady state operation.
It would further be desirable to provide an auto-starting system for a lamp of the mentioned type including the feature of turning off power to the lamp if an overload condition is sensed and further, one in which power supplied to the lamp is regulated to be within a narrow range.